Abstract
We extend the approach to representation of partial information based on orderings on objects from sets to multisets. We characterize orderings arising under closed- and open-world assumptions and analyze their complexity. In contrast to the set case, where orderings are first-order definable and are thus expressible in standard database query languages, the orderings on bags are not expressible in standard bag languages. We give an example of a query on nested relations whose inexpressibility in the extension of relational algebra to nested objects cannot be proved by reduction to the first-order case.
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